In general, the WLAN service has been distributed in the country and abroad on the basis of IEEE 802.11b standard having a maximal data rate of 11 Mbps by using 2.4 GHz band frequencies. 802.1x is used for user authentication, and standardization of 802.11i for security and 802.11e for quality of service are in progress.
Here, interworking of the WLAN and the cellular network must be considered in order to provide a flexible service by overcoming coverage limits of an access point (AP). Accordingly, a lot of interests have been made on the activation of the WLAN service which has been distributed worldwide and the interworking of the WLAN and cellular network.
The most reliable method for interworking the WLAN and the cellular network is to use a mobile IP. A general method using the mobile IP disconnects a physical layer on a session and re-connects it. It takes a few seconds according to an existing protocol.
However, in order to provide a real time service such as image or voice communication, the disconnection and re-connection time must be reduced as much as possible. That is, when a terminal moves from one network to another, a re-authentication and re-access process for the terminal must be performed in a short time. Therefore, there are strong demands for a roaming method between the WLAN and the cellular network which can reduce a time consumed to support mobility.